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5.21 Climate Change

than the growth rate from the beginning of continuous direct measurements in 1960 to 2005
(ESRL 2017).

Greenhouse Gases
GHGs, as defined by the United States Environmental Protection Agency (USEPA) and
California Air Resources Board (CARB), include CO2, methane (CH4), nitrous oxide (N2O),
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). GHGs
are global pollutants and are therefore unlike criteria air pollutants such as ozone (O3),
particulate matter (respirable particulate matter with a diameter of 10 microns or less
[PM10] and fine particulate matter with a diameter of 2.5 microns or less [PM2.5]), and toxic
air contaminants (TACs), which are pollutants of regional and local concern (see Section
5.11, Air Resources, of this EIR). While pollutants with localized air quality effects
have relatively short atmospheric lifetimes (generally on the order of a few days), GHGs have
relatively long atmospheric lifetimes, ranging from one year to several thousand years. Long
atmospheric lifetimes allow for GHGs to disperse around the globe. Therefore, GHG effects
are global, as opposed to the local and/or regional air quality effects of criteria air pollutant
and TAC emissions.

Global warming potential (GWP) is a term used to indicate, on a pound for pound basis, how
much a gas will contribute to global warming relative to how much warming would be
caused by the same mass of CO2. As the baseline for measuring GWP, CO2 is considered to
have a GWP equal to one. CH4 and N2O are substantially more potent than CO2 with GWPs of
25 and 298, respectively. (Prior values of 21 and 310 were from the Intergovernmental Panel
on Climate Change [IPCC] second assessment report; CARB has adopted the current values
from the IPCC’s fourth assessment report.) Carbon dioxide equivalent (CO2e) is a quantity
that enables all GHG emissions to be considered as a group despite their varying GWP. The
GWP of each GHG is multiplied by the quantity of that gas to produce CO2e.

The Greenhouse Effect

In a greenhouse, sunlight enters through the glass panels, and the heat from the sunlight is
then trapped inside the structure. The Earth’s atmosphere acts like a greenhouse by allowing
sunlight in, but traps some of the heat that reaches the Earth’s surface. When solar radiation
from the sun reaches the Earth, much of it penetrates the atmosphere and ultimately reaches
the Earth’s surface. This solar radiation is absorbed by the Earth’s surface and is then re-
emitted as heat in the form of infrared radiation. Whereas the GHGs in the atmosphere let
solar radiation through, GHGs trap infrared radiation, resulting in the overall warming of the
Earth’s surface.1 This phenomenon is referred to as the “greenhouse effect”.

Concentrations of major greenhouse gases, such as CO2, CH4, N2O, and water vapor (H2O)
have been naturally present for millennia at relatively stable levels in the atmosphere, and
act to keep temperatures on Earth hospitable. Without these GHGs, the earth’s temperature
would be too cold for life to exist. With increased human industrial activity, concentrations
of certain GHGs have grown dramatically. In the absence of major industrial human activity,

1 Infrared radiation has longer wavelengths than does solar radiation. GHGs reflect radiation with longer wavelengths.
As a result, instead of escaping back into space, GHGs reflect much infrared radiation (i.e., heat) back to Earth.

R:\Projects\PAS\CEN\000306\Draft EIR\5.21 ClimateChange-051117.docx 5.21-5 Centennial Project
Draft EIR